The present invention relates to wireless communications and, in particular, to methods and systems for scanning and locking onto a control channel in a wireless communications system.
In cellular telephone and other wireless communication systems, wireless terminals often need to xe2x80x9clock ontoxe2x80x9d or xe2x80x9ccamp onxe2x80x9d a control channel when powering up and/or when transitioning between base stations. By xe2x80x9clockingxe2x80x9d or xe2x80x9ccampingxe2x80x9d it is meant that the wireless terminal acquires synchronization with the control channel. In many situations, the wireless terminal will not know the location (operating frequency) of the best available control channel on which it could camp. Thus, the wireless terminal may have to locate such a control channel. This may be accomplished by xe2x80x9cscanningxe2x80x9d the operating frequencies corresponding to known channels in the communications system to locate a control channel which is considered acceptable. The process of scanning channels to locate such a control channel is sometimes referred to as an Intelligent Roaming (xe2x80x9cIRxe2x80x9d) search or as a Power-Up Scan.
In many cellular telephone and other wireless communications systems, the wireless terminals may access communications channels operated by multiple service providers. For instance, in wireless communications systems operating under the North American TIA/EIA-136 cellular telephone standard, wireless terminals may access up to eight different frequency bands, each of which contain hundreds of separate channels. Typically, each of the eight frequency bands (and hence all the channels in that band) are operated by a different service provider. In many situations, wireless terminals may only be allowed to operate on a subset of the eight frequency bands, in that some of the bands are forbidden. Moreover, the cost to the wireless terminal (or to the wireless plan the terminal is using) may vary depending upon the frequency band used. In light of these variations in cost and availability, service providers are categorized under TIA/EIA-136 into five different groupsxe2x80x94Home, Partner, Favored, Neutral and Forbidden service providersxe2x80x94which are listed in priority order from the most preferred to the least preferred. As a result, the goal of an IR search typically is not just to locate a control channel, but to locate and camp on a control channel of the highest priority service provider available. Wireless terminals operating under the TIA/EIA-136 standard include an Intelligent Roaming Database or xe2x80x9cIRDBxe2x80x9d which contains lists of Home, Partner, Favored and Forbidden service providers.
In many wireless communications systems, the IR search is performed as a series of separate scans of selected operating frequencies. For instance, the system standard for TIA/EIA-136 wireless communications systems specifies an IR Search that may include as many as four separate stages. In the first stage of the IR search, the wireless terminal scans a set of Private Operating Frequencies (xe2x80x9cPOFxe2x80x9d). These private operating frequencies are the frequencies on which certain private or residential systems operate (at least in some geographic areas) to which the wireless terminal is allowed access. If a Digital Control Channel xe2x80x9cDCCHxe2x80x9d that is part of such a private or residential system having an acceptable receive signal strength (xe2x80x9cRSSxe2x80x9d) level is identified in the POF scan, then the wireless terminal xe2x80x9clocks ontoxe2x80x9d the channel by entering the DCCH camping state.
If a private or residential system DCCH is not found during the POF scan, the wireless terminal proceeds to the second stage of the IR search. In this stage, control channels listed in a Digital Control Channel History Table (xe2x80x9cDHTxe2x80x9d) that is maintained in memory by the wireless terminal are scanned. The DHT contains a non-redundant listing as to the band and DCCH channel number of the control channels (excluding control channels on POF""s) on which the wireless terminal has most recently camped. The DHT can contain multiple entries for a particular frequency band. The DHT is provided in TIA/EIA-136 compatible wireless terminals to speed-up the search for a control channel during an IR search, as, in many cases, an acceptable control channel corresponding to a highest priority service provider will be listed in the DHT. Under TIA/EIA-136, a control channel identified during the DHT scan is only considered acceptable for camping if it has at least the minimum receive signal strength and if it is a channel operated by either a Home or a Partner service provider.
If an acceptable control channel is not identified during the DHT scan, the IR search continues to its third stage, which is referred to as the wideband scan. The wideband scan is typically the most involved of the channel scans. During the wideband scan, each of the frequency bands specified in the IRDB is scanned, and the wireless terminal once again seeks to identify a DCCH operated by a Home or Partner service provider that has a minimum receive signal strength level. In lieu of scanning all of the hundreds of channels that are part of each frequency band, during the wideband scan each frequency band is typically divided into a number of xe2x80x9csub-bandsxe2x80x9d or xe2x80x9cprobability blocksxe2x80x9d, and the channels having the highest received signal strength in each sub-band/block are selected for scanning.
During the wideband scan, the wireless terminal will examine the first control channel scanned in each frequency band to determine if the frequency band is operated by a Home, Partner, Favored, Neutral or Forbidden service provider. If the frequency band is operated by a Favored, Neutral or Forbidden service provider, the scan of the band is typically aborted at that point, and the frequency band along with the highest received signal strength channels from each sub-band or probability block in a band operated by a Favored or Neutral service provider are added to a xe2x80x9cRe-Scan Listxe2x80x9d that identifies the frequency bands that are scanned during the fourth and final stage of the IR search.
If an acceptable control channel is not identified during the wideband scan, the IR search proceeds to the fourth or re-scan stage. During this stage, the wireless terminal scans each of the frequency band-channel pairs included in the Re-Scan List. During the fourth stage of the IR search, if the wireless terminal identifies a control channel having at least the minimum received signal strength that is operated by a Favored or Neutral service provider, the wireless terminal attempts to enter the DCCH camping state on that channel.
In embodiments of the present invention, methods and systems are provided by which a wireless terminal may select a control channel from a plurality of channels. These methods and systems may be used by a wireless terminal to select a control channel on which it will camp, remain idle, or otherwise utilize. In particular embodiments of the present invention, a first set of operating frequencies are scanned to locate a control channel that satisfies a first set of selection criteria. During this scan, the wireless terminal also identifies channels that fail to satisfy the first set of selection criteria but that have an increased probability of satisfying a second set of selection criteria. If during the scan of the first set of operating frequencies a control channel is located that satisfies the first set of selection criteria, that control channel may be selected.
If a control channel is not selected, the wireless terminal may determine an order in which a second set of operating frequencies are to be scanned. This order may be based, at least in part, on the channels identified during the scan of the first set of operating frequencies as having an increased probability of satisfying the second set of selection criteria. The second subset of operating frequencies may then be scanned in the determined order to locate a control channel that satisfies the second set of selection criteria which, if located, may then be selected.
As will be appreciated by those of skill in the art, the present invention may be embodied as methods or systems.